The present invention relates to a substantially planar magnetic sensor made in particular on an integrated circuit, this magnetic sensor being of the fluxgate type and being typically intended to be fitted to a magnetometer for detecting, in a plane, magnetic fields of very low value, for example for medical applications. This magnetometer is then preferably made using CMOS techniques, with its associated electronic circuit integrated in the substrate on which the sensor is made.
The publication xe2x80x9cIntegrated Planar Fluxgate Sensor With Amorphous Metal Corexe2x80x9d by Messrs. L. Chiesi, J. A. Flaganan, B. Jannosy, and R. S. Poponic of the Swiss Federal Institute of Technology of Lausanne (Switzerland), presented at the xe2x80x9cEuroSensors XIxe2x80x9d conference in Poland, 1997 can be cited as the state of the art.
This publication describes a planar magnetic micro-sensor integrated on a silicon substrate, which is shown in FIG. 1 annexed as an illustration of the prior art.
As is seen in FIG. 1, this magnetic sensor, or xe2x80x9cfluxgatexe2x80x9d sensor is made, by CMOS integration techniques, to a silicon micro-substrate 1, or xe2x80x9csilicon chipxe2x80x9d, formed by a plate of parallelepiped shape.
This micro-sensor is integrated on the large top face 2 of parallelepiped substrate 1, the latter being positioned, with respect to the external magnetic field to be measured Hext, so that large face 2 is practically coplanar to external field Hext.
The actual magnetic sensor includes a metal tape 3 of amorphous metal material through which flows an excitation current carried by two aluminium wires 5, 6 respectively welded to each end of tape 3. This excitation current is an ac current of frequency f, of triangular shape, which generates a magnetic field, of the same frequency, which, because of the non-linear nature of the curve B-H (magnetic flux-magnetic field) of the tape of amorphous ferromagnetic core 3, periodically saturates ferromagnetic material 3.
The sensor includes two coplanar detection coils 7 and 8, mounted in series and in opposition, i.e. in differential arrangement, which are each placed respectively at one end of amorphous ferromagnetic tape or core 3, where they thus each measure the leakage field at these respective ends.
Because of the non-linear permeability of amorphous ferromagnetic core 1, the voltage detected by each coil 7 or 8 contains harmonics of excitation frequency f, of which only the even harmonics are interesting since they are proportional to the external magnetic field Hext to be measured. Generally, detection is thus carried out on frequency 2f.
This known device has the drawback, on the one hand, of not allowing external field Hext to be measured along two orthogonal directions, without having to change the relative position of the sensor, and on the other hand, of lacking sensitivity. Moreover its output voltage is dependent on the frequency of the excitation current and its current consumption is relatively high.
An object of the invention is to provide a magnetic sensor which is similar to the sensor according to FIG. 1, but which does not have the aforecited drawbacks and is greatly improved as regards its performance, in particular its lower current consumption and its ability to detect external magnetic fields Hext of very low intensity and to detect magnetic fields along two orthogonal directions.
The invention therefore concerns a substantially planar magnetic sensor, made on a semiconductor substrate, this sensor including:
one or more magnetic cores which are substantially planar and made on a large planar face of the substrate;
at least one excitation circuit; and
at least one detection circuit formed by at least two planar detection coils also made on this large face of the substrate, characterised in that said excitation circuit is formed by a single planar coil of substantially square external shape, and in that said magnetic core or cores are long-limbed cores which are arranged, in a Greek cross, along the two diagonals of the geometric square defined by the outer contour of said planar excitation coil.
According to a preferred embodiment, there is only one said magnetic coil and it is monoblock and has the shape of a Greek cross arranged along said diagonals. Advantageously, in this case, the detection circuit is formed by four detection coils which are respectively placed at the four free ends of said magnetic core in the shape of a monoblock Greek cross. Preferably, the material forming the magnetic core or cores is amorphous.